Optical scanning and imaging systems based on dual pulsed laser systems

ABSTRACT

The invention relates to scanning pulsed laser systems for optical imaging. Coherent dual scanning laser systems (CDSL) are disclosed and some applications thereof. Various alternatives for implementation are illustrated, including highly integrated configurations. In at least one embodiment a coherent dual scanning laser system (CDSL) includes two passively modelocked fiber oscillators. The oscillators are configured to operate at slightly different repetition rates, such that a difference δf r  in repetition rates is small compared to the values f r1  and f r2  of the repetition rates of the oscillators. The CDSL system also includes a non-linear frequency conversion section optically connected to each oscillator. The section includes a non-linear optical element generating a frequency converted spectral output having a spectral bandwidth and a frequency comb comprising harmonics of the oscillator repetition rates. A CDSL may be arranged in an imaging system for one or more of optical imaging, microscopy, micro-spectroscopy and/or THz imaging.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation application of U.S. application Ser.No. 13/754,491, filed Jan. 30, 2013, which is a continuation applicationof Ser. No. 13/316,740 filed Dec. 12, 2011, which is a continuationapplication of Ser. No. 12/399,435 filed Mar. 6, 2009, now issued asU.S. Pat. No. 8,120,778, Feb. 21, 2012 the disclosure of which isincorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to scanning pulsed laser systems for opticalimaging.

2. Description of the Related Art

Dual pulsed laser systems comprising two modelocked lasers operating attwo slightly different repetition rates f₁ and f₂, such thatδ=|(f₁−f₂)|<<f₁ and δ=|(f₁−f₂)|<<f₂, are useful instruments for therapid interrogation of optical response functions of widely varyingelectronic and opto-electronic devices such as photoconductive switchesand integrated circuits. Additionally the use of dual pulsed lasersystems has also been suggested for THz imaging as disclosed in U.S.Pat. No. 5,778,016 and U.S. Pat. No. 6,396,856 to Sucha et al.

The use of dual modelocked lasers can be replaced for probing theoptical response functions by implementing dual electronic circuitsystems, as has been suggested in U.S. Pat. No. 5,748,309 by van derWeide. The approach has some benefit for the interrogation of thespectral dependence of signal transmission in the THz spectral range.Two pulsed signal sources also operating at two slightly differentrepetition rates f₁ and f₂ were disclosed, which produce emission in theTHz spectral range made up of frequency lines comprising pure harmonicsof the two repetition rates. Detection of the beat signal at δ, 2δ, . .. nδ is then used to infer the signal transmission at the harmonics ofthe repetition rate f₁, 2f₁, . . . nf₁. Note that in this scheme a beatsignal at a difference frequency as low as δ is used, which is notideal, since δ typically has a small value where acoustic noise cancorrupt the signal.

The use of mode locked lasers was again later disclosed by Keilmann etal., in ‘Time domain mid-infrared frequency-comb spectrometer’, Opt.Lett., vol. 29, pp. 1542-1544 (2004), who suggested the use of a dualscanning laser system for Fourier Transform Spectroscopy (FTS) and theanalysis of the spectral transmission of materials in the infraredspectral range.

In order to improve the scan rate of dual laser scanning FTS, Keilmannet al., in International Patent Application Publication WO2007/045461,further suggested to dither the repetition rate of one laser versus theother using techniques similar to the ones described in the '016 patent.

The use of lasers for spectroscopy has also been suggested by Haensch etal. in U.S. Pat. No. 7,203,402, where a single frequency comb laserbased on a mode-locked laser was used for the measurement of certainproperties of optical elements. Here the measurement was performedeither simultaneously or sequentially at each individual frequency lineof the comb laser.

A frequency comb laser was recently also combined with a conventionalFourier transform spectrometer to obtain an improved signal/noise ratiofor spectral measurements (J. Mandon et al., ‘Fourier transformspectroscopy with a laser frequency comb’, in Nature Photonics, 2009)

Prior dual scanning laser systems have a number of limitations whenapplied to spectroscopy. The low repetition rate of implemented lasersources leads to excessively long data acquisition times. and thetechniques for signal generation in the near IR to mid-IR spectral rangeare relatively cumbersome. Systems implemented with bulky solid-statelasers are not well suited for instrumentation applications and requirea large components count. Other systems (P. Giaccari et al., ‘ActiveFourier-transform spectroscopy combining the direct RF beating of twofiber-based mode-locked lasers with a novel referencing method’, Opt.Express., vol. 16, pp. 4347 (2008)) and (I. Coddington et al., “CoherentMultiheterodyne Spectroscopy Using Stabilized Optical Frequency Combs,”Phys. Rev. Lett. 100, 13902 (2008)) provide only very limited spectralcoverage.

SUMMARY OF THE INVENTION

In the following we refer to dual scanning laser systems that exploitthe discrete frequency spectrum, i.e. the comb spectrum, of modelockedlasers but that do not require or do not rely on precision comb controlinside the laser oscillator as coherent dual scanning lasers, CDSLs.

Here we disclose a new CDSL for applications in spectroscopy,micro-spectroscopy, microscopy, Fourier transform spectroscopy (FTS),optical and THz imaging, and/or similar applications. The CDSLs arebased on modelocked fiber lasers designed for operation at highrepetition rates allowing for large scanning speeds. Efficientspectroscopic measurements are enabled by the implementation of lownoise, phase controlled fiber lasers, which are designed to providebroad spectral coverage via the implementation of nonlinear spectrallybroadening optical elements. Various compact designs are described. Invarious embodiments a reduction of component count is furtheraccomplished via simultaneous use of nonlinear spectral broadeningelements and the use of appropriate time delays between the lasers.

We further disclose the use of highly nonlinear waveguides inconjunction with coherent supercontinuum generation for generating anoptical output from the visible to the mid-infrared spectral region.Difference frequency generation (DFG) produces output in the mid-IRspectral region and simplifies the implementation of FTS. DFG eliminatesvariations of the carrier envelope offset frequency external to thelaser cavity and thus produces an output spectrum comprising trueharmonics of the laser repetition rates.

In conjunction with photoconductive antennas spectral emission in theTHz spectral region can be obtained.

In order to use difference frequency generation effectively, the modelocked lasers can be configured with two outputs each. Amplifiers can befurther implemented to amplify those outputs. Supercontinuum generationcan then be implemented for spectral broadening of these fiber laseroutputs. Difference frequency generation can be implemented betweenspectral components of the supercontinuum or between a spectralcomponent of the supercontinuum and another fiber laser output.

Nonlinear signal interference in nonlinear frequency broadening elementsfrom overlapping pulses can be eliminated by using separate nonlinearfrequency broadening elements for each laser. Alternatively, an opticaldelay line can be inserted at the output of the CDSL to produce aninterference signal only from pulses that do not overlap in anynonlinear optical elements. Electronic gating can further be implementedfor optimum signal conditioning.

In at least one embodiment the carrier envelope offset frequencies incoherent dual scanning femtosecond modelocked fiber lasers can beadjusted by control of various intra-cavity optical elements such asintra-cavity loss, saturable absorber temperature, fiber temperature andfiber grating temperature. In some embodiments carrier envelope offsetfrequency control can be averted by the implementation of DFG.

In at least one embodiment the carrier envelope offset frequencies andrepetition rates in coherent dual scanning femtosecond modelocked fiberlasers can further be controlled by phase locking the two lasers toexternal cavities.

In at least one embodiment the carrier envelope offset frequencies andrepetition rates in coherent dual scanning femtosecond modelocked fiberlasers can further be controlled by phase locking the two lasers to twoexternal single-frequency lasers.

In another embodiment the difference in carrier envelope offsetfrequencies and the repetition rates in coherent dual scanningfemtosecond modelocked fiber lasers can further be controlled by phaselocking the two lasers to one external single-frequency laser.

For improved spectral resolution coherent dual scanning femtosecondmodelocked fiber lasers can also be constructed with lasers where therepetition rate of one laser is an approximate harmonic of therepetition rate of the other laser.

The noise of the carrier envelope offset frequencies can be minimized byan appropriate adjustment of the intra-cavity laser dispersion, and thepulse width injected into the supercontinuum fibers.

Any drift in carrier envelope offset frequency between the two lasers inthe CDSLs can be monitored and corrected for by external optical means.Also, an f−2f interferometer can be implemented for carrier envelopeoffset frequency monitoring.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating an example of a CDSL.

FIG. 2 is a schematic diagram of an optically integrated dispersioncompensator, and non-linear frequency conversion section as used forsupercontinuum generation.

FIG. 3 is a schematic diagram of a CDSL as used for optical imagingapplications.

FIG. 4 is a schematic diagram of a CDSL designed with a reduced numberof components.

FIG. 5 is a schematic diagram of yet another CDSL based on carrierenvelope offset frequency monitoring.

FIG. 6 a is a schematic diagram of an intra-cavity assembly of amodelocked fiber oscillator for resistive heating of an intra-cavityfiber Bragg grating for carrier envelope offset frequency control.

FIG. 6 b is a schematic diagram of an intra-cavity assembly of amodelocked fiber oscillator for modulating the pressure applied to anintra-cavity fiber Bragg grating for carrier envelope offset frequencycontrol.

FIG. 6 c is a schematic diagram of an assembly including an intra-cavitymodulator as used for modulating the intra-cavity loss of a modelockedlaser for carrier envelope offset frequency control.

FIG. 6 d is a schematic diagram of an intra-cavity assembly of amodelocked fiber oscillator for modulating the residual pump powerimpinging on an intra-cavity saturable absorber for carrier envelopeoffset frequency control.

FIG. 7 is a plot of the RF spectrum of a carrier envelope offsetfrequency locked Yb fiber laser operating at a repetition rate of 1 GHzmeasured after a nonlinear f−2f interferometer.

FIG. 8 is a plot of the spectral output of Yb fiber laser based coherentsupercontinuum source operating at a repetition rate of 1 GHz.

FIG. 9 is a schematic diagram of a dual scanning laser system which islocked to two external cavities for repetition rate and carrier phasecontrol.

FIG. 10 is a schematic diagram of a dual scanning laser system which islocked to two narrow linewidth lasers for repetition rate and carrierphase control.

FIG. 11 is a schematic diagram of an ultra-compact dual scanning lasersystem which uses one external narrow linewidth laser for repetitionrate and carrier phase control.

DETAILED DESCRIPTION OF THE INVENTION

This description first discusses some aspects of mode-locked lasers andfrequency comb generation particularly related to CDSL and applicationsthereof. Examples of applications of such lasers for IR spectroscopy andTHz imaging are included.

Modelocked lasers with fixed optical frequency spectra comprising a setof equidistant optical frequency lines are typically also referred to asfrequency comb lasers. The optical frequency spectrum of a frequencycomb laser can be described by S(f)=f_(ceo)+mf_(rep), where m is aninteger, f_(ceo) is the carrier envelope offset frequency and f_(rep) isthe repetition rate of the laser. The amplitudes of the individualfrequency lines in fact sample the optical envelope spectrum at discretepoints f_(ceo)+mf_(rep) in optical frequency space.

Frequency comb lasers were described in U.S. Pat. No. 6,785,303 toHolzwarth et al., where the control of the pump power of a modelockedlaser in conjunction with electronic feedback loops is used to stabilizef_(ceo) and thereby to stabilize the location of all individualfrequency lines that comprise the optical frequency spectrum. In astandard modelocked laser f_(ceo) is not controlled and therefore onlythe separation of all individual frequency lines is stable, apart from aslow drift of f_(r) due to cavity length fluctuations. As disclosedabove the spectral separation corresponds to the repetition rate f_(rep)of the modelocked oscillator, which is generally a frequency in the MHzregime, and in various embodiments described herein more preferablyabout 1 GHz or even higher. The exact location of the lines inside thefrequency spectrum varies randomly. However, the optical spectrum of afrequency comb laser and a modelocked laser can have the same envelopefunction. Also even if f_(ceo) is not controlled, the optical spectrumof a modelocked laser comprises a number of discrete frequency lines.

When operating two frequency comb lasers at slightly differentrepetition rates f_(rep) and f_(rep)+δ respectively, and whenoverlapping the output of the two lasers on a detector various beatfrequencies can be observed in the RF domain. When further ensuring thatfor both lasers frequency teeth of order m are the most proximate, theRF spectrum includes harmonic frequencies mδ+Δf_(ceo), (m+1)δ+Δf_(ceo),(m+2)δ+Δf_(ceo) . . . with amplitudes from the geometrical mean of theamplitudes at optical frequencies mf+f_(ceo1) and m(f+δ)+f_(ceo2) whereΔf_(ceo)=f_(ceo1)−f_(ceo2). The parameter f_(rep)/δ is the scalingfactor that scales the RF frequencies to the optical frequencies. Forexample for δ=10 Hz and f_(rep)=1 GHz, we obtain a scaling factor of10⁸; an intensity measured in the RF domain at 1 MHz corresponds to anoptical frequency of 10¹⁴ Hz. Δf_(ceo) can be selected to further lowerthe RF frequency where measurements need to be performed in order toobtain the amplitude of the optical frequencies. The RF frequency atwhich measurements are to be performed can be changed by ensuring thatfor the two lasers frequency teeth of order m and n respectively aremost proximate. In this case the beat frequencies are given by(m−n)f_(r)+mδ+Δf_(ceo); (m−n)f_(r)+(m+1)δ+Δf_(ceo);(m−n)f_(r)+(m+2)δ+Δf_(ceo . . . .)

The need for a comb laser with a fixed optical frequency spectrum forspectroscopic measurements can be relaxed via the implementation ofcorrection techniques that monitor the drift of the frequency lineswithin the spectral envelope, as recently discussed by P. Giaccari etal., ‘Active Fourier-transform spectroscopy combining the direct RFbeating of two fiber-based mode-locked lasers with a novel referencingmethod’, Opt. Express., vol. 16, pp. 4347 (2008).

Alternatively, the drift of the individual frequency lines can beeliminated by adding a nonlinear frequency conversion section after amodelocked oscillator. For example when implementing differencefrequency generation between the red and blue part of the modelockedlaser spectrum, it is well known that the individual frequency linesoccur precisely at true harmonics of the laser repetition rateindependent of the value of f_(ceo). As previously discussed, we referto dual scanning laser systems that take advantage of the discretefrequency spectrum of modelocked lasers but that do not rely onprecision phase or f_(ceo) control inside the laser oscillator generallyas CDSLs.

For any instrumentation applications of modelocked lasers, mode-lockedfiber lasers have several advantages over both mode-locked bulk solidstate lasers and mode-locked diode lasers. Mode-locked fiber lasersoffer typically superior noise properties compared to mode-locked diodelasers and can be packaged in smaller spaces than mode-locked bulk solidstate lasers. Mode-locked fiber lasers can be produced with excellentthermal and mechanical stability. Passively mode-locked fiber lasers inparticular can be constructed with few and inexpensive opticalcomponents, suitable for mass production, as disclosed in U.S. Pat. No.7,190,705 to Fermann et al. and Ser. No. 11/546,998 to Hartl et al. U.S.Pat. No. 7,190,705 is hereby incorporated by reference in its entirety.Additionally, the use of dual pulsed laser systems has also beensuggested in THz imaging as disclosed in U.S. Pat. No. 5,778,016 andU.S. Pat. No. 6,396,856 to Sucha et al. The '016 and '856 patents alsodisclose various techniques and configurations for controlling relativeand absolute timing drift of mode-locked lasers. U.S. Pat. Nos.5,778,016 and 6,396,856 are hereby incorporated by reference in theirentirety.

The dispersion compensated fiber lasers as disclosed in '705 provide forthe construction of low noise frequency comb sources. Also disclosedwere designs of fiber lasers operating at repetition rates in excess of1 GHz.

Low-noise operation of fiber lasers minimizes their timing jitter,allowing optimized control of the timing of the pulses. The '705 patentdisclosed the first low noise fiber-based frequency comb source. Lownoise operation was obtained by controlling the fiber cavity dispersionin a certain well-defined range. Low noise operation of fiber frequencycomb sources is generally required in order to minimize the noise of thecarrier envelope offset frequency f_(ceo) of the laser to a negligiblelevel, and also to facilitate measurement and control of f_(ceo).

Some examples of fiber-based CDSL systems are disclosed below.Implementations providing for high repetition rate, low-noise, and ahigh-level of integration are described. Non-linear spectral generationand various implementations for phase-control lead to stable outputsignals in the near-IR range, thereby providing benefits for IRspectroscopy and THz imaging applications.

FIG. 1 schematically illustrates a coherent dual scanning laser system100 (CDSL) according to an embodiment. In this example two mode-lockedoscillators 110 a, 110 b having slightly different repetition rates areutilized to provide input pulse trains. A pulse train from eachoscillator is amplified and split into first and second optical paths.The pulses in each path are conditioned with a dispersion compensator.An intermediate non-linear frequency conversion section generates asupercontinuum along a first path, which is then combined with the pulsetrain in the second path using a non-linear frequency converters forDFG. The DFG outputs corresponding to each of the oscillators 110 a, 110b are then combined to produce CDSL output.

Referring to FIG. 1, the system comprises two oscillators 110 a-, 110-b(oscillator 1 and oscillator 2), which preferably generate pulses thatcan be compressed to the femtosecond (fs) time scale. Preferablyoscillators 110 a,110 b are implemented using Er, Yb or Tm oscillatorsoperating at repetition rates of about 250 MHz or higher. Suchoscillators were for example described in U.S. Pat. No. 7,190,705 toFermann et al. and Ser. No. 11/546,998 to Hartl et al. as well as inU.S. provisional application U.S. 61/120,022, entitled “HighlyRare-Earth-Doped Optical Fibers for Fiber Lasers and Amplifiers” to Donget al., which is incorporated herein by reference. Various examplesdisclosed in the '022 application include highly rare earth doped gainfibers having pump absorption of up to about 5000 dB/m, and gain perunit length in the range of 0.5-5 dB/cm. Various dopant concentrationsreduce Yb clustering thereby providing for high pump absorption, largegain, with low photodarkening. Such rare-earth doped fibers provide forconstruction of short cavity length fiber lasers, and for generation ofhigh energy ultrashort pulses at a repetition rate exceeding 1 GHz. Suchconfigurations provide for high signal to noise operation of CDSLs. Byway of example, other fiber configurations having high pump absorptioncompared to conventional silica fibers, for example absorption of300-1500 dB/m at 976 nm are also disclosed in U.S. Ser. No. 11/693,633,entitled “Rare earth doped and large effective area optical fibers forfiber lasers and amplifiers”, now published as U.S. patent applicationpub. no. 2008/0069508. U.S. Ser. No. 11/693,633 is hereby incorporatedby reference in its entirety.

The output of the oscillators is preferably passed through opticalisolators (not shown) to minimize their sensitivity to back reflections.The repetition rates of the two oscillators can be monitored using twotab-couplers inserted into the output of the two oscillators, whichdirect a small fraction of the output of the oscillators onto twodetectors (not shown), which provide signals representative of therepetition rate to controller 101.

The oscillators can be constructed to operate at respective repetitionrates of f and f+δ, where δ<<f. Alternatively, the repetition rate ofthe second oscillator can be selected to be nf+δ, where n is an integer.The difference between their repetition rates δ, or (n−1)f+δ for thecase of widely dissimilar repetition rates, can then be controlled byrepetition rate control element 101 comprising phase-locked loops and anintra-cavity transducer introduced into one of the oscillators. Such anintra-cavity transducer can be a mirror mounted on a piezoelectricelement or a fiber heating element, for example as discussed in U.S.Pat. No. 7,190,705 to Fermann et al. and U.S. patent application Ser.No. 11/546,998 to Hartl et al. The oscillators may emit nearly chirpfree pulses or slightly chirped pulses. Preferably any chirped pulsesemitted from the oscillators 110-a,110-b have nearly-identical chirp.Preferably the power of both oscillators can be adjusted over somerange, for example by a variable attenuator.

The outputs of the oscillators are coupled to two fiber amplifiers120-a, 120-b. The fiber amplifiers are preferably cladding pumped. Suchcladding pumped amplifiers are discussed in U.S. Pat. No. 7,190,705 toFermann et al. Also cladding pumping via optical star-couplers asdescribed by Dong et al. in Ser. No. 61/120,022 “Highly Rare-Earth-DopedOptical Fibers for Fiber Lasers and Amplifiers” can be implemented andis not further discussed here. Preferably the dispersion in bothoscillators-amplifier propagation paths is matched.

In the example of FIG. 1 optical signal pulses output from each fiberamplifier are split into two arms: arms 125 a-1, 125 a-2 opticallyconnected to amplifier 120 a and oscillator 110 a, and arms 125 b-1, 125b-2 optically connected to amplifier 120 b and oscillator 110 b. Opticalfiber couplers are preferred, and splitting ratios between 5/95 and50/50 can be implemented. Each arm may be implemented in an all-fiberconfiguration as illustrated. In some embodiments at least a portion ofan arm may be constructed with bulk components.

Dispersion compensation is carried out in the optical paths of each armto compensate for dispersion, for example with a series of dispersioncompensating elements forming a dispersion compensator. At least aportion of the arms may be constructed from identical components,including the various dispersion compensating elements. Dispersioncompensating elements can include optical elements for pulse compressionto provide high-quality femtosecond pulses, and may provide completedispersion compensation or produce slightly negatively or positivelychirped pulses at their output. When complete dispersion compensation isused, the output pulses are nearly transform limited.

A dispersion compensator can comprise several different fiber elements,and may be implemented with an integrated “all-fiber” design as will befurther discussed below with respect to FIG. 2. For example, a firstfiber element can comprise a positive dispersion fiber designed tospectrally broaden the output of the amplifier, and at least a secondfiber element for dispersion compensation and for compressing thespectrally broadened output to near the bandwidth limit. Preferably thepulse compressing fiber element comprises a dispersion compensatingfiber or a photonic crystal fiber with a central air-hole to minimizethe nonlinearity of the dispersion compensation stage. Pulse compressionvia higher-order soliton compression in one or more negative dispersionfiber elements can be implemented. Also, bulk optic dispersioncompensation elements such as grating, prism or grism pairs may be used.Preferably, the pulses are compressed to a pulse width less than about500 fs, more preferably to a pulse width about less than 300 fs, andmost preferably to a pulse width less than about 100 fs.

Optical pulses propagating in arms 125 a-2, 125 b-2 are also frequencyconverted in a non-linear frequency conversion section having frequencyconversion elements 130 a, 130 b. Frequency conversion elements 130 a,130 b can include optical elements for supercontinuum generation toprovide pulses with a spectral bandwidth of at least a substantialfraction of an optical octave, and substantially broader than an outputspectrum of oscillators 110 a, 110 b. Frequency conversion elements 130a,130 b generate a broadband spectrum, for example a spectrum extendinginto the near mid-IR region, for example extending from the near IR tothe range of at least about 3-5 μm, or up to about 10-20 μm.

In various implementations a frequency conversion section preferablycomprises a highly nonlinear fiber, a periodically poled LiNbO3 (PPLN)waveguide, a silicon waveguide or any other suitable nonlinearwaveguide. An element may also be optically patterned or periodically oraperiodically poled or have periodic variations of the 2^(nd)—ordernonlinearity along its length. The frequency conversion sections 130 a,130 b in each arm generate an optical supercontinuum spectrum that canextend into the mid-IR when using highly nonlinear fluoride orchalcogenide waveguides. Supercontinuum generation in nonlinearwaveguides was described in U.S. patent application Ser. No. 11/546,998to Hartl et al. and is not further discussed here. As known insupercontinuum generation, the fundamental frequency comb structure fromthe oscillators is preserved. The additional spectral output generatedcomprises individual frequency teeth separated in frequency by therepetition rate of the laser. However, the injection of pulses shorterthan 300 fs, and more preferably shorter than 100 fs, reduces theincoherent background between individual frequency teeth of thesupercontinuum spectrum. The incoherent background is undesirablebecause it reduces the signal contrast in CDSLs. The influence ofincoherent background noise to the comb contrast is described in N.Newbury and W. Swann, “Low-noise fiber-laser frequency combs,” Journalof the Optical Society of America B 24, 1756-1770 (2007) which isincorporated herein by reference.

Difference frequency generation (DFG) is carried out in non-linearfrequency conversion sections 140 a,140 b by mixing the dispersioncompensated output signal pulses from paths 125 a-1,125 b-1 with thecorresponding dispersion compensated and frequency converted outputs,including a supercontinuum generated in a section of arms 125 a-2, 125b-2. The outputs are mixed in frequency converters 140 a, 140 b.Frequency converters 140 a, 140 b are preferably configured withnonlinear crystals such as LiNbO₃, GaAs, GaSe, GaP or any other suitablenonlinear crystal. These nonlinear crystals may also be periodicallypoled, optically patterned or have periodic variations of their 2^(nd)order nonlinearity along their length. Nonlinear waveguides can also beimplemented. Frequency filters and polarization controllers can furtherbe included upstream of DFG elements 140 a, 140 b and are not separatelyshown. The output from DFG elements is combined via beam splitter 150and directed to the output 160.

In various embodiments the optical signal pulses output from theamplifiers are further directed through an optical isolator beforeinjection into the dispersion compensation and frequency conversionstages. Appropriate time delays between the two oscillators are furtherintroduced to ensure pulse overlap in the DFG elements and beam splitter150. Such time delays can be introduced by well known methods ofcontrolling the fiber lengths and free space propagation paths and arenot separately shown here.

An all-fiber construction of a dispersion compensator and non-linearfrequency section of each arm provides some benefits. One benefit ofusing a highly nonlinear fiber for frequency conversion sections 130 a,130 b is that the amplifier stages 120 a, 120 b, the dispersioncompensation elements and the frequency conversion section 130 a, 130 bcan all be spliced together as schematically illustrated in FIG. 2.Various elements are shown in FIG. 2 which may be used in each arm,particularly in 125 a-2,125-b 2 where both dispersion compensation andsuper-continuum generation are performed. Polarization maintaining fibercomponents can also be implemented, or alternatively polarizationcontrollers (not shown) can be used to optimize the polarization statefor supercontinuum generation. A fiber pig-tailed isolator (not shown)preferably isolates the output of the amplifier from unwanted backreflections.

In the example illustrated in FIG. 2, a dispersion compensation fiber215 is spliced onto a length of transfer fiber 220 on each end, whichtransforms the fundamental mode of the fiber to match the mode in theadjacent fiber, such as any amplifiers 120 a,120 b providing inputs, ora highly nonlinear fiber 230 providing an output as shown in FIG. 2. Thehighly non-linear fiber, configured with the arrangement as shown inFIG. 2, may also be utilized in frequency conversion sections, forexample sections 130 a, 130 b. A transfer fiber 220 can comprise morethan one piece of fiber and can also comprise fiber optic tapers.

The output pulses of the amplifier emitted from transfer fiber 220 arethen compressed in a length of dispersion compensation fiber 215. Alength of photonic crystal fiber can be used, but any other type offiber with suitable nonlinear and linear properties can also beimplemented for pulse compression. Both linear and nonlinear amplifiers,such as similariton amplifiers as described in U.S. Pat. No. 6,885,683to Fermann et al., can be implemented. When nonlinear amplifiers areimplemented the oscillator power-levels can be preferably adjusted. Forexample similariton amplifiers produce positively chirped pulses, whichcan be compressed to near the bandwidth limit in a length of photoniccrystal fiber as discussed in U.S. Pat. No. 7,414,780 to Fermann et al.

The highly nonlinear fiber 230 is then used for supercontinuumgeneration. Highly nonlinear fibers based on silica, are discussed inU.S. Pat. No. 7,496,260, entitled “Ultra High Numerical Aperture OpticalFibers” to Dong et al., which is hereby incorporated by reference in itsentirety. In various embodiments non-silica-fibers with improved IRtransparency can be used. For example nonlinear fluoride, bismuth,telluride or chalcogenide fibers can be implemented. Such IRtransmitting fibers can transmit wavelengths up to around 20 μm and arecommercially available. Because the melting temperature of mid-IRtransmission fiber is typically much smaller than the meltingtemperature of silica fibers, optical lens arrangements can further beused to couple light from the dispersion compensating fiber to thehighly nonlinear fiber in order to avoid complicated splicing of fiberswith largely different melting temperatures.

An optical imaging system that includes a CDSL is shown in FIG. 3. Herea beam splitter is inserted after the output of the CDSL and splits theoutput along two optical paths. The beam splitter directs a fraction ofthe output along a first path onto detector D2, which is used to measurea reference spectrum, representing the output of the CDSL as a functionof wavelength. The sample under test is inserted into the second path.By dividing the spectrum measured with detector D1 by the spectrummeasured with detector D2 an accurate absorption spectrum of the testsample can be obtained. Such two detector schemes are well known instandard Fourier transform spectroscopy to eliminate spectral variationsand temporal drifts of the light source in absorption measurements. Inorder to obtain the spatial distribution of the sample absorption and toperform imaging, an optical scanner 310 such as a commercially availablegalvanometer mirror system is further inserted into the second beam pathof the output of the CDSL. In some embodiments the sample under test canbe mounted on a movable stage. In various embodiments a combination ofbeam motion and movement of the sample may be utilized. The output ofthe CDSL is then focused onto the sample under test with a microscopeobjective 325 or other suitable beam delivery optics. The transmittedlight is detected with a detector D1. In various embodiments onereference spectrum is obtained with detector D2. Alternatively, detectorD2 can be omitted and the reference spectrum can be obtained by takingthe sample out of the beam path 2. In some embodiments reflected lightmay be detected, or a combination of transmitted and reflected light. Inorder to improve the signal to noise ratio in the IR, cooling of thedetector can also be implemented. For example liquid nitrogen cooledHgCdTe (MCT) detectors can be implemented that are commerciallyavailable with detection bandwidths up to 100 MHz. Also filter wheels(not shown) can be inserted anywhere in the beam path to select certainoptical frequency ranges. An image is then obtained by monitoring RFspectra for each image point and by appropriately relating those RFspectra to optical transmission or reflection spectra.

The detector D1 monitors beat frequencies in the RF domain. Due toscaling of the optical frequencies to the RF frequencies with a scalingfactor f_(rep)/δ in a CDSL we can interpret the function of the CDSL asrepresenting a frequency grid in the RF domain for scaling RF to opticalfrequencies; each optical frequency is uniquely mapped onto an RFfrequency, with a 1:1 correspondence. Difference frequency generation asillustrated in FIG. 1 cancels the carrier envelope offset frequenciesexternal to the lasers. Because the carrier envelope offset frequenciesof the two lasers after the DFG stages are zero, the relation betweenoptical f_(opt) and RF beat frequencies f_(rf) is given by

f _(opt) =f _(rf) ×f _(rep)/δ  (1),

where the minimum RF frequency RF_(min) that contains information aboutsignal transmission at optical frequencies is given by mδ. Note thatsince m is a large number (of order 10⁴ or higher), RF_(min) can be ofthe order of MHz.

An alternative embodiment of a CDSL is shown in FIG. 4. Here thecomponent count is reduced by using two oscillators operating atslightly different repetition rates. The oscillator outputs arecombined, and coupled into a common propagation path. The components inthe common propagation path may be similar or identical to the onesdescribed with respect to FIG. 1. In this example one amplifier 420, oneintermediate supercontinuum generation section 430 and one DFG section440 are used. The output of amplifier 420 is split into arms 425 a,425 bin a manner similar to that illustrated in FIG. 1. DFG is obtained frommixing the dispersion compensated output of arm 425 a with the outputfrom the supercontinuum generator configured in arm 425 b. Non-linearcrystal 440 provides for DFG, as discussed with respect to FIG. 1. Theoutput of the system is detected with detectors D1 and D2, where D2 isused to obtain a reference spectrum and D1 is used to measure theabsorption of the test sample. Additional optical components forscanning can also be incorporated as in the example of FIG. 3.

In order to avoid signal degradation due to nonlinear interactions attimes when the output pulses of the two oscillators overlap in time atthe DFG section, the detectors can be electronically gated to benon-responsive at those times. In order to obtain an interference signalat times when the output pulses of the two oscillators do not overlap intime, an optical delay line 470 can be incorporated in front of thedetector D1 and D2 (or at the output of the CDSL) as shown. In at leastone embodiment a delay line based on a Mach-Zehnder interferometer isutilized, although other types of delay lines such as a Michelsoninterferometer with unbalanced arm lengths can also be implemented. Thetime delay line conveniently produces a time delay of a fraction of thecavity round trip time of the two lasers, where preferably this fractionis 50%. When recording an interferogram with an optical delay line asmall penalty results from increased background signal and increasedshot noise, but that penalty is greatly offset by the benefit of reducedcomponent count of the system. Unwanted nonlinear pulse interactions dueto potential pulse overlap in other parts of the system can be furtheravoided by an appropriate control of the pig-tail lengths from the twooscillators.

Another example of a CDSL is shown in FIG. 5. As described with respectto FIG. 1 the system also comprises a repetition rate controller 101,two oscillators 110 a, 110 b (oscillator 1 & oscillator 2) andamplifiers 120 a,120 b. The system configuration is very similar to thesystem described with respect FIG. 1, but the DFG sections areeliminated. The output of the two oscillators propagates along twodifferent propagations paths and is injected into two separate fiberoptic amplifiers 520 a,5200 b. Preferably the amplifier and oscillatorexhibit overlapping gain spectra. Preferably both oscillators emit chirpfree pulses or pulses having nearly identical chirp. Preferably thedispersion along the two propagation paths is matched. Preferably thepower of both oscillators can be adjusted over some range, for exampleby a variable attenuator. The output of the amplifiers is furtherdirected through an optical isolator (not shown) before injection intotwo dispersion compensators and frequency conversion sections similar indesign to the arms 125 a-2, 125 b-2 described with respect to FIG. 1. Inthis example the frequency conversion sections generate two preferablyvery broad supercontinuum spectra that can span an octave or more andcan extend into the mid-IR when, for example, using highly nonlinearfluoride or chalcogenide waveguides. Several frequency conversionsections as well as intermediate amplifiers can be concatenated andfrequency conversion stages based on PPLN waveguides or siliconwaveguides can also be used.

In contrast to the system described with respect to FIG. 1, a portion ofthe output of the two generated supercontinua is diverted to two phasedetection and control units 540 a, 540 b. Phase detection can forexample be conveniently performed with an f−2f interferometer asdiscussed in U.S. Pat. No. 7,190,705 to Fermann et al. and Ser. No.11/546,998 to Hartl et al. Such f−2f interferometers are therefore notfurther discussed here. The f−2f interferometer produces an RF beatsignal corresponding to f_(ceo) which is fed back to the oscillators forf_(ceo) control via a feedback loop. The f_(ceo) of both oscillators canbe kept within an RF filter bandwidth by a frequency lock by feedbackloops. For optimum precision of the feedback loop a phase locked loopcan be implemented, but other feedback loops can also be implemented.

As discussed in U.S. Pat. No. 7,190,705 the temperature of anintra-cavity fiber grating inside a modelocked fiber oscillator can beused for carrier envelope phase control. Alternatively, as alsodiscussed in '705, an external pressure can be applied to the fibergrating and pressure variations can be used for carrier envelope phasecontrol.

In FIG. 6 a variety of techniques for carrier envelope phase control ina fiber oscillator are illustrated. In FIG. 6 a a piece of intra-cavityfiber 601 is shown that contains a fiber Bragg grating for cavitydispersion control. In order to control the carrier-envelope offsetfrequency, the outside of the fiber grating is gold coated and a currentis passed through the coating. The temperature of the grating can thusbe controlled via resistive heating in the gold coating which in turnleads to a rapid modulation of the carrier-envelope offset frequency,which can then be stabilized via a feedback loop in conjunction with anf−2f interferometer.

In FIG. 6 b a piezo-electric transducer (PZT) 602 applies pressure toone side of the fiber, which can also be modulated and used forintra-cavity carrier-envelope offset frequency control. The use of a PZTallows for faster feedback control compared to a resistive heater.

Even faster carrier-envelope offset frequency control can beaccomplished via an intra-cavity acousto-optic modulator (AOM) 603 asshown in FIG. 6 c By changing the drive voltage to the AOM the lossinside the fiber oscillator can be rapidly modulated, which in turnleads to a modulation of the carrier-envelope offset frequency.

In FIG. 6 d the carrier-envelope offset frequency is controlled via amodulation of the residual pump power that is impinging on theintra-cavity saturable absorber. This is accomplished by inserting apolarizer in front of the saturable absorber and modulating thepolarization of the pump light 604. The polarization of the pump lightcan be modulated in a variety of ways; essentially lossless and rapidpolarization modulation is possible by passing the pump light through alength of polarization maintaining fiber which is coiled onto a PZT drumand exciting both axes of the polarization maintaining fiber equallywith linearly polarized pump light.

Other means for carrier-envelope offset frequency control can also beimplemented; for example the temperature of the intra-cavity saturableabsorber can be modulated. Various combinations may also be implemented.Moreover, the feedback systems of FIG. 6 may also comprise multiplefeedback loops for independent measurement and control of the carrierenvelope offset frequency.

In FIG. 7 the corresponding RF spectrum of a carrier envelope offsetfrequency locked Yb fiber laser operating at a repetition rate of 1.04GHz measured after a nonlinear f−2f interferometer is shown. The RFspectrum shows a peak at 1 GHz corresponding to the repetition rate ofthe laser and two peaks at 210 and 830 MHz corresponding to the carrierenvelope offset frequency.

FIG. 8 illustrates an example of a supercontinuum spectrum generatedwith a carrier-envelope phase locked Yb fiber laser operating at arepetition rate of 1 GHz. Here the supercontinuum was generated in ahighly nonlinear optical fiber. The supercontinuum spectrum was recordedfrom one propagation path of the fiber system as shown in FIG. 5.

Rather than controlling f_(ceo) with an f−2f interferometer, f_(ceo) canalso be controlled by referencing the frequency comb of a mode lockedlaser to the Fabry-Perot resonances of a passive cavity. This techniquehas some benefits: no octave spanning continuum generation is requiredand relatively small power levels which can be provided by a portion ofthe oscillator power are sufficient. This method is described in R.Jason Jones and Jean-Claude Diels “Stabilization of Femtosecond Lasersfor Optical Frequency Metrology and Direct Optical to Radio-Synthesis”PRL 86, p. 3288 (2001) and in R. Jason Jones et al. “Precisionstabilization of femtosecond lasers to high-finesse optical cavities”.Phys. Rev. A 69, 051803 (2004) which is hereby incorporated by referencein its entirety.

An embodiment utilizing external cavities is shown in FIG. 9. Theoscillator 110 a, 110 b outputs are combined and coupled into a singlepropagation path, and amplified with fiber amplifier 920, as discussedabove. A portion of each oscillator output is also directed to referencecavities 940 a, 940 b, and stabilized at two separated spectral regionsto a reference cavity. In this example two reference cavities 940 a, 940b with slightly different round trip times are shown. Both cavities arepreferably in close thermal and mechanical contact for all thermally andmechanically induced fluctuations being in common mode. A configurationwith one external cavity is also possible. When using only one cavity abirefringent element incorporated into the cavity provides for twodifferent round-trip times along two polarization axes, where those twoaxes are in turn locked to each laser. Such an implementation is notseparately shown.

Gratings 950 a,950 b direct two spectral regions of the oscillatoroutput to two different detectors, which are then used to lock the twodifferent oscillator comb teeth to two different resonances of theexternal cavities, which may be configured as passive cavities, or withfeedback control. With the use of one or two external cavities all fourdegrees of freedom, namely f_(ceo) of both lasers, as well as f_(rep)and δ are referenced to the cavity modes. In a preferred embodiment aPound Dreyer Hall scheme is used for locking the oscillators to thereference cavities. The Pound Dreyer Hall scheme requires theimplementation of additional phase modulators (not shown) in the fiberpig-tails that transport the signal to the external cavities. Instead ofseparate phase modulators, phase modulation can also be implementedintra-cavity by for example modulating one cavity end mirror. The PoundDreyer Hall scheme is well known in the state of the art for lockingmode-locked fiber lasers to external cavities and is not furtherexplained.

As an alternative to using passive cavities as stable references forrepetition rate and carrier phase control of CDSLs cw reference lasers1080 a,1080 b can be used as shown in FIG. 10. The oscillator 110 a, 110b outputs are also combined and coupled into a single propagation path,and amplified with fiber amplifier 1020, as discussed above.Additionally, two stable, single frequency lasers are used forstablization. Such single frequency lasers are preferably based onsemiconductor or fiber lasers and are commercially available. Thefrequencies of both single frequency lasers are different and areselected such that they both overlap with the spectra of the mode-lockedlasers, preferably at the low and high frequency parts of the spectra.The frequency comb of each mode-locked laser can now be phase-locked attwo comb teeth to the stable cw-lasers by feedback control to f_(ceo)and f_(rep) of both lasers, fixing two frequencies of each comb andtherefore stabilizing f_(ceo) and f_(rep). When the stabilization isarranged such that for each mode-locked laser a different number of combteeth are between the two fixed comb teeth, their repetition rates aredifferent which is required for CDSL. An implementation of thisstabilization method is described in I. Coddington, et al. “CoherentMultiheterodyne Spectroscopy Using Stabilized Optical Frequency Combs,”Phys. Rev. Lett. 100, 13902 (2008), which is included by reference.

A compact and highly integrated configuration of a CDSL is shown in FIG.11. The oscillator 110 a, 110 b outputs are combined and coupled into asingle propagation path, and amplified with fiber amplifier 120, asdiscussed above. A portion of the amplified output is sampled anddirected to detector D1. Alternatively, the output of the oscillators orthe amplified outputs of the oscillators may be directed to twodifferent detectors, such an implementation is not separately shown. Asingle frequency laser is used for repetition rate and carrier phasecontrol. In this scheme one of the frequency teeth of oscillator 110 ais locked to the single frequency laser via a phase-locked loop (PLL1)and one of the frequency teeth of oscillator 110 b is locked to thesingle frequency laser via another phase-locked loop (PLL2). To ensurephase-locking the cavity length of each laser can be modulated. Afterphase locking of these two frequency teeth, the expression for thefrequency spectra of the two oscillators can be written as

mf _(rep) f _(ceo1) =f _(y) +f _(b1)  (2)

n(f _(rep)+δ)+f _(ceo2) =f _(y) +f _(b2),  (3)

where f_(y) is the frequency of the single-frequency laser and f_(b1),f_(b2) are the RF beat frequencies to which the frequency teeth of thetwo oscillators are locked; δ is the difference between the repetitionrates of the lasers. F_(rep) and δ can further be locked to two more RFreference signals by controlling, for example, the pump current to thelasers. We can further evaluate n and m by setting for example the laserconditions such that mf_(rep)=f_(y)+f_(b1), where f_(y)+f_(b1) isobtained from an external calibration using for example a wavemeter. Thesame procedure can be implemented to obtain n. In the following weassume for simplicity n=m. Taking the difference of eqs (2) and (3) wethen obtain

nδ+Δf _(ceo) =Δf _(b).  (4)

Since Δf_(b) and δ are known and n, m can be a obtained using thecalibration procedure described above, from eqs. (2) or (3), we canevaluate Δf_(ceo) to great precision.

It can then be easily shown that for Δf_(ceo)≠0 the frequency grid inthe RF domain is frequency shifted compared to eq. (1) and the relationbetween optical f_(opt) and RF frequencies f_(rf) is modified as

f _(ops)=(f _(rf) −Δf _(ceo))f _(rep) /δ+f _(ceo1)≈(f _(rf) −Δf _(ceo))f_(rep)/δ,  (5)

Here we can simplify eq. (5) since f_(ceo)<<f_(opt). In this exampleΔf_(ceo) is stabilized rather than the individual carrier envelopeoffset frequencies in order to obtain an accurate RF frequency grid forthe measurement of optical frequencies.

To obtain the best long-term precision for frequency measurements with aCDSL system, both oscillators are preferably packaged in close proximityin order to equalize any thermal fluctuations of laser parametersbetween the two lasers. Also, the single frequency reference laser isalso preferably packaged with good thermal control.

Additionally, the system can be set up with amplifiers and nonlinearfrequency conversion sections for increased spectral coverage. Atemporal delay line 470 can be inserted in front of the detectors D2,D3to detect pulse interference without temporal pulse overlap in thenonlinear stages. Also, two detectors D2, D3 can be used, where onedetector is used for detection of a reference spectrum and the other isused to determine the absorption characteristics of a sample.

The system of FIG. 11 is beneficial for commercial applications becauseit can operate with a few components and a high level of opticalintegration.

The embodiments described above may be combined in various ways toproduce alternative implementations. Many possibilities exist, andvarious modifications may be made based on specific applications. Forexample, a non-linear frequency conversion section may be configuredwith at least one non-linear fiber amplifier to broaden a spectrum.

Referring back again to FIG. 5, a phase control unit can also bereplaced with a phase monitor unit. For example a phase monitor unit cancomprise a fiber optic tap splitter (inserted in front of the amplifier)which diverts some of the oscillator light, and selects two narrowspectral lines from the oscillator spectra to monitor the carrierenvelope offset frequency and repetition rate evolution. The carrierenvelope offset frequency evolution can also be monitored after theamplifier or the first frequency conversion stage, but usage of theoscillator signal provides for the lowest noise. Such phase monitorunits were discussed in Giaccari et al. and are not separately describedhere. As an alternative to such a phase monitor unit also two f−2finterferometers can be used which monitor the carrier envelope offsetfrequency of the two oscillators. As discussed with respect to FIG. 3the output of the CDSL can be passed through an optical sample and canalso be directed to a detector, where beat frequencies in the RF domainare observable. Due to scaling of the optical frequencies to the RFfrequencies with a scaling factor f_(rep)/δ in CDSLs, we can interpretthe function of the scanning dual laser system as providing a frequencygrid in the RF domain for scaling RF to optical frequencies, i.e. eachoptical frequency is mapped onto a RF frequency. When locking thecarrier envelope offset frequencies of the two lasers to differentvalues eq. (5) can be used to obtain an accurate RF frequency grid forthe measurement of optical frequencies.

More complex modifications of the RF frequency grid and the relationbetween optical and RF frequencies are obtained for small continuousvariations of Δf_(ceo) as well as δ. By the implementation of a phasemonitor unit the appropriate corrections of the RF frequency grid can becalculated in order to obtain an accurate conversion of RF to opticalfrequencies. Such corrections to the RF frequency grid were discussed byGiaccari and are not further described here. Similar corrections canalso be applied when monitoring Δf_(ceo) with an f−2f interferometer.Since the f−2f interferometer allows a direct reading of f_(ceo) for ofeach oscillator using RF techniques, Δf_(ceo) can be easily computed andthe optical frequencies can be calculated using equation (5).

The imaging arrangement as discussed with respect to FIG. 3 can also beextended to the THz range. As discussed by Yasui et al., in Appl. Phys.Lett., vol. 88, pp. 211104-1 to 3 (2006) a THz comb is generated by aphoto-conductive emitter excited by a femtosecond laser. The sameapplies also when generating THz pulses via optical rectification in anelectro-optic crystal. Moreover, the frequency comb in the THz frequencyrange comprises pure harmonics of the laser repetition rate. Thus twoslightly offset THz frequency combs can be generated by directing theoutput of a CDSL system onto an electro-optic crystal or aphoto-conductive emitter. A system for generating THz frequency combsand imaging in the THz spectral range can thus be constructed similar tothe implementation shown in FIGS. 3 and 5, where the frequencyconversion section and the phase control sections are omitted and thenonlinear frequency conversion section is replaced with an electro-opticcrystal such as for example GaP, GaSe, periodically poled LiNbO₃,optically patterned GaAs or a photo-conductive antenna. Appropriate THzoptics can then be used for imaging the THz radiation onto a sample,which is conveniently placed on a movable stage for optical scanning. Invarious embodiments the scanner may be omitted, in part because of thepresent limited availability of scanners suitable for use over the THzfrequency range. An appropriate detector such as a photo-conductiveantenna can then monitor the RF beat signal from which the THz spectrumcan be inferred using an RF analysis of the detected photo-current inthe detector as discussed by Yasui et al.

Thus the inventors have described CDSLs and some applications thereof,and various alternatives for implementation including highly integratedconfigurations.

In at least one embodiment a coherent dual scanning laser system (CDSL)includes two passively modelocked fiber oscillators. The oscillators areconfigured to operate at slightly different repetition rates, such thata difference δf_(r) in repetition rates is small compared to the valuesf_(r1) and f_(r2) of the repetition rates of the oscillators. The CDSLsystem also includes a non-linear frequency conversion section opticallyconnected to each oscillator. The section includes a non-linear opticalelement generating a frequency converted spectral output having aspectral bandwidth and a frequency comb comprising harmonics of theoscillator repetition rates.

In various embodiments:

a frequency conversion section includes an output section that receivesand combines multiple input frequencies and generates a spectral outputat a difference frequency thereof, and the system includes anintermediate non-linear frequency conversion section between at leastone oscillator and the output section, the intermediate sectiongenerating a broadband spectrum having a bandwidth substantially greaterthan an oscillator spectrum.

a CDSL is arranged in a measurement system that utilizes spectralinformation, and a spectral output is utilized to probe a physicalproperty of a test sample with spectral components within the spectralbandwidth.

a CDSL is arranged in an imaging system for one or more of opticalimaging, microscopy, micro-spectroscopy and/or THz imaging.

a CDSL based measurement system may include an element for opticalscanning.

a phase-locked loop controls the difference in repetition rates betweenthe oscillators.

an RF spectrum analyzer generates an output at RF frequencies related tothe optical frequencies with a conversion factor(f_(r1)+f_(r2))/2δf_(r).

mode locked fiber oscillators comprise an Nd, Yb, Tm or Er fiberoscillator.

at least one fiber amplifier is included for amplifying one or moreoscillator outputs.

an integrated, all-fiber, dispersion compensator and non-linearfrequency conversion section is included, the integrated sectioncomprising one or more of a highly nonlinear fiber, a photonic crystalfiber, a dispersion compensating fiber and/or a fiber having a centralair-hole.

a system includes a bulk optical element for dispersion compensation,including at least one of a grating pair, prism pair and/or grism pair,wherein dispersion compensation comprises pulse compression.

a nonlinear frequency conversion section comprises a differencefrequency generator.

a non-linear frequency section includes a supercontinuum generatordisposed downstream of at least one oscillator.

a mode locked fiber oscillator generates pulses at a repetition rategreater than about 250 MHz.

In at least one embodiment a coherent dual scanning laser systemincludes two passively modelocked fiber oscillators. The oscillators areconfigured to operate at slightly different repetition rates, such thata difference δf_(r) in repetition rates is small compared to the valuesf_(r1) and f_(r2) of the repetition rates of two oscillators. The CDSLalso includes a non-linear frequency conversion section opticallyconnected to each oscillator, the section comprising a non-linearoptical element generating a frequency converted spectral output havinga spectral bandwidth and a frequency comb structure with a frequencyseparation equivalent to the oscillator repetition rates. The nonlinearfrequency conversion section produces a spectral output substantiallybroader than the spectral output from each oscillator.

In various embodiments:

a means for monitoring the difference in the carrier envelope offsetfrequencies of the two lasers is included, wherein information generatedby the monitoring means provides a 1:1 correspondence between RFfrequencies and optical frequencies.

a correspondence is represented with a 1:1 mapping of said RFfrequencies to optical frequencies.

an f−2f interferometer is included for carrier envelope offset frequencycontrol of each laser.

a feedback system is included for stabilizing the difference in thecarrier envelope offset frequencies of the two oscillators.

carrier envelope offset frequency information generated by the feedbacksystem is used to generate a frequency grid in the RF domain that has aone to one correspondence to a frequency grid in the optical domain.

a feedback system includes a single-frequency reference laser.

two reference cavities are utilized for carrier envelope offsetfrequency control of each oscillator.

one reference cavity is utilized for carrier envelope offset frequencycontrol of each oscillator.

two single-frequency reference lasers are utilized for carrier envelopeoffset frequency control of each oscillator.

a non-linear frequency conversion section comprises a highly non-linearfiber.

a ratio of a repetition rate to the difference in repetition rates is atleast about 10⁶, and may be in the range of about 10⁶ to about 10⁹.

The repetition rates f_(r1), f_(r2), and ratio of a repetition rate tothe difference in repetition rates are sufficiently high to convert anRF frequency to an optical frequency.

At least one embodiment includes a system for imaging in the THzspectral range. The imaging system includes a coherent dual scanninglaser system (CDSL) having two passively modelocked fiber oscillators.The modelocked oscillators are configured to operate at slightlydifferent repetition rates, such that a difference δf_(r) in repetitionrates is small compared to the values f_(r1) and f_(r2) of therepetition rates of the oscillators. The system includes a materialemitting THz radiation in response to an output of said CDSL, and adetector responsive to said THz radiation.

In at least one embodiment a coherent dual scanning laser systemincludes two passively modelocked oscillators generating at least twotrains of short optical pulses. The oscillators are configured tooperate at slightly different repetition rates, such that a differencein repetition rates δf_(r) is small compared to the values f_(r1) andf_(r2) of the repetition rates of the oscillators. The system includes abeam combiner for spatially combining trains of short optical pulses topropagate along a common optical path downstream of the beam combiner. Anon-linear optical element is included for spectrally broadening atleast one train of said short optical pulses propagating along thecommon optical path. A dual arm interferometer is configured withdifferent arm lengths so as to detect interference between pulse trainswhen the pulses are not temporally overlapping in time prior to enteringthe interferometer. In various embodiments an arm length differencecorresponds to approximately half the cavity round trip time of saidoscillators.

In at least one embodiment a coherent dual scanning laser systemincludes two passively modelocked fiber oscillators generating twoseparate trains of short optical pulses. The oscillators are adjusted tooperate at slightly different repetition rates, such that a differenceδf_(r) in repetition rates is small compared to the values f_(r1) andf_(r2) of the repetition rates of the oscillators. A feedback systemstabilizes the difference in the carrier envelope offset frequencies ofthe two oscillators, and the feedback system includes a single-frequencylaser. A beam combiner spatially combines trains of short optical pulsesto propagate along a common optical path downstream of the beamcombiner. The system includes a non-linear optical element forspectrally broadening at least one train of short optical pulsespropagating along a common optical path. A dual arm interferometer isconfigured with different arm lengths so as to detect interferencebetween pulse trains when the pulses are not temporally overlapping intime prior to entering the interferometer.

Thus, while only certain embodiments have been specifically describedherein, it will be apparent that numerous modifications may be madethereto without departing from the spirit and scope of the invention.Further, acronyms are used merely to enhance the readability of thespecification and claims. It should be noted that these acronyms are notintended to lessen the generality of the terms used and they should notbe construed to restrict the scope of the claims to the embodimentsdescribed therein.

What is claimed is:
 1. A laser based system for generating pulse pairswith a time varying time delay, said system comprising: an opticalsource, said source comprising: at least one mode locked oscillator,said source generating optical pulses at a time-varying repetition rate,said source generating an output comprising said pulse pairs derivedfrom said at least one mode locked oscillator; a repetition ratemodulator configured to modulate said repetition rate at a modulationrate; and an optical reference comprising at least one optical elementconfigured for generating a reference signal for measurement of at leastthe time delay between two pulses of said pulse pair as a function oftime.
 2. The laser based system according to claim 1, wherein saidsource further comprises: a first beam divider downstream of one modelocked oscillator, said beam divider configured to propagate the outputof said one mode locked oscillator along two optical paths havingdifferent propagation lengths; and a beam combiner configured torecombine said pulses propagating along said two optical paths.
 3. Thelaser based system according to claim 1, wherein said system comprisesat least one optical element configured to provide for a measurement ofa difference in carrier envelope phase between pulses of said pulsepair.
 4. The laser based system according to claim 1, wherein saidrepetition rate modulator modulates a cavity length of a correspondingmode locked oscillator.
 5. The laser based system according to claim 1,said optical reference comprising at least one cw reference laserconfigured to record beat signals between said pulse pairs and said atleast one reference laser.
 6. The laser based system according to claim1, said optical reference comprising at least one narrow bandpass filterconfigured for selection of narrow spectral lines to record beat signalsbetween said pulse pairs.
 7. The laser based system according to claim1, wherein the system comprises a spectral broadening stage down-streamof said source or said at least one mode locked laser.
 8. The laserbased system according to claim 1, wherein said system comprisesdetectors to detect an interferogram between said pulse pairs.
 9. Thelaser based system according to claim 1, wherein said system furthercomprises means for generating sampling points at equidistant opticalpath length differences between the pulses comprising said pulse pair.10. The laser based system according to claim 1, wherein said sourcecomprises two passively mode locked fiber lasers.
 11. The laser basedsystem according to claim 10, wherein outputs of each passively modelocked fiber laser are combined such that said pulse pairs propagatealong common optical paths.
 12. The laser based system according toclaim 10, further comprising a fiber amplifier configured to amplifysaid pulses pairs propagating in a common optical path.
 13. A systemcomprising: a laser source generating a pulse train of pulse pairs witha time varying time delay, said source comprising: at least one modelocked laser; and a repetition rate modulator configured to control therepetition rate of said mode locked laser, and to control the time delayof pulses generated with said source; a first beam divider downstream ofa mode locked oscillator, said beam divider configured to propagate theoutput of said mode locked oscillator along two optical paths havingdifferent propagation lengths; and a beam combiner configured torecombine said pulses propagating along said two optical paths.
 14. Thesystem according to claim 13, further comprising: a target receivingsaid time delayed and recombined pulses.
 15. The system according toclaim 13, said system configured for spectroscopy and/or imaging in theTHz spectral range.
 16. The system according to claim 13, furthercomprising a detector to detect beat signals between said time delayedpulses in the RF domain from which a corresponding THz spectrum isobtainable.
 17. The system according to claim 13, said measurement ofsaid THz spectrum is based on a frequency grid in the RF domain that hasa 1:1 correspondence to a frequency grid in the THz domain, therebyallowing an RF frequency to be scaled to a THz frequency.
 18. The systemaccording to claim 13, further comprising: material emitting THzradiation in response to said at least one pulse; and a detectorresponsive to said THz radiation.
 19. The system according to claim 13,wherein said system is arranged in an imaging system for one or more ofoptical imaging, microscopy, micro-spectroscopy and THz imaging.
 20. Thesystem according to claim 13, wherein said repetition rate modulatorcomprises intracavity optical elements disposed in a cavity containingsaid at least one mode locked oscillator.
 21. A system comprising: alaser source generating a pulse train of pulse pairs with intra-pairtime varying time delay, said source comprising: at least one modelocked laser; and a repetition rate modulator configured to control therepetition rates, and to control the intra-pair time delay of pulsesgenerated with said source; a beam splitter directing respective pulsesof said pulse pair along two different propagation directions; a beamcombiner comprising a second beam splitter for recombining the pulses ofsaid pulse pair; and a phase monitor to monitor the pulse separationbetween said two time delayed pulses.